The present invention relates in general to turbocharger designs and lubrication of the turbocharger thrust and journal bearings. More specifically the present invention relates to a modification of the thrust bearing to incorporate an oil reservoir in order to provide initial lubrication during engine start up, especially cold weather starting.
Turbocharger bearings are subject to excessive wear during initial engine start up. This is due to the lack of lubricating fluid at the point of contact between the stationary and rotating components of the bearing system prior to full lubrication pressure being built up in the engine after start up. An aggravating factor for the turbocharger bearing system is the fact that the oil is typically siphoned away from the turbocharger as soon as the engine is shut down. Since the turbocharger may continue to coast down in speed for a period of time approaching approximately 30 seconds, the bearings are left in an extremely dry condition for the next engine start up.
Available data, reports and failure analysis indicate that thrust or journal bearing failures may comprise as much as 50% of turbocharger warranty failures. Much of this damage is due to the delay in oil delivery to the turbocharger after high speed engine starts at cold ambient conditions.
In recent years increasingly stringent engine design requirements have meant higher and higher rotary speeds for the typical diesel engine turbocharger. Even at engine idle conditions, these high rotor speeds combine with increased aerodynamic thrust loads to create much higher rates of wear in the bearing system during the few seconds after an engine start before the turbocharger receives lubricating oil. The combination of smaller turbine casings, higher rotary speeds and higher aerodynamic loads at idle has aggravated this failure mode.
In a typical turbocharger installation, the turbocharger is mounted at a relatively high position on the engine so that it can receive exhaust gases directly from the exhaust manifold and supply compressed air directly to the intake manifold or aftercooler. The oil supply however, typically arrives at the turbocharger bearing housing from a location farther down in the engine block where pressurized oil is present. Due to these relative points of attachment, a siphoning action occurs immediately after the engine is stopped and this siphoning action drains oil from the turbocharger oil supply line back into the engine.
While the engine is running, the oil pump supplies oil to all components of the engine and all oil passages are filled with oil. Immediately following engine shutdown therefore, oil is present at some high level within the engine. As the oil drains down within the engine it soon drops below the level of the turbocharger oil supply rifle. At this point a suction is created by the difference in oil head between the level in the turbocharger and the continually falling oil level in the engine. Air is drawn into the turbocharger oil line at the journal bearings and thrust bearing lubrication points. Air continues to flow into the oil supply system of the turbocharger until the siphon is broken by the accumulated air trapped in the oil supply line. This has been measured to be a period of time similar to that required for the turbocharger to coast to a stop following engine shutdown. Because of the siphoning action virtually no oil is allowed into the thrust system after engine shutdown, even though the turbocharger continues to spin for approximately 30 seconds afterwards
Because of the continued spinning of the turbocharger without additional oil flow to replenish the bearing system, most of the oil present in the bearing system is pumped out, leaving the bearings exposed to damage during the next engine start up.
The present invention solves this thrust bearing lubrication problem by creating an oil reservoir in the thrust bearing which drains oil into the journal bearings and thrust bearing. Oil is retained by capillary action within the bearings and is available for the next engine startup, even if the engine is not started for several days.